WO-A-95/26499 describes a holographic sensor. The sensor comprises a holographic support medium and, disposed throughout its volume, a hologram. The support medium interacts with an analyte, resulting in a variation of a physical property of the medium. This variation induces a change in an optical characteristic of the holographic element, such as its polarisability, reflectance, refractance or absorbance. If any change occurs whilst the hologram is being replayed (e.g. using incident broad band, non-ionising electromagnetic radiation), then a colour change, for example, may be observed using an optical detector. The optical detector may be a spectrometer or simply the human eye.
WO-A-99/63408 describes an alternative method of producing a holographic sensor. A sequential treatment technique is used, wherein the polymer film is made first and sensitive silver halide particles are incorporated subsequently. These particles are introduced by diffusing soluble salts into the polymer matrix and reacting them with halide ions and a sensitising dye, to form an insoluble light-sensitive precipitate. The holographic image is then recorded.
PCT/GB04/00976 describes how holographic sensors can be produced using a technique known as “silverless double polymerisation”.
Holographic sensors have been proposed for subcutaneous use, e.g. to detect glucose. However, the holograms do not generally reflect light of sufficient intensity to penetrate through the skin and be detected. This is mainly because of problems of light scatter.
The support medium of a conventional holographic sensor may comprise a cross-linker such as N,N′-methylenebisacrylamide (MBA).
Hochstrasser et al, Analytical Biochemistry (1988) Volume 173, pages 412-423, report investigating several different cross-linkers in acrylamide gels, to limit the background binding of silver to the gel when carrying out silver staining of the gel. They found that any cross-linker with free amides such a methylenebisacrylamide (MBA) tended to cause a lot of background staining. They postulated that the free amides on MBA were responsible for interacting with the silver and binding it to the polymer. They found that bisacryloylpiperazine (BAP) which is a tertiary amide (unlike MBA and most other bisacrylamide cross-linkers, which are secondary amides) did not exhibit this random binding and high silver background during silver staining. This was thought to be due to the nitrogens in BAP being unable to interact with silver.